Some good points, but I'll stick to my original position.
I have so seen too many cases where the main OS partition gets knocked out taking all of the data with it. Keeping the data partition separate has worked well for me.
There are downsides to a full volume restore. If your server wasn't booting for whatever reason, you would have to lose that data that was in the main partition. E.g. rolling back a day means you would lose that data for the day you did. If you kept it in a separate volume, there's a good chance that you would have the data and your server would be back to normal. Windows backup lets you recover volumes individually.
And if a potential follow up point is that you could keep multiple backups of the data (AC and shared docs, faxes, whatever) in that event. If that is the case then using partitioning should be easy enough.
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